class_docstr: Internal Var (with '__' prefix and won't appear in SSA) will ALWAYS have
  `sym_val == builtin.unknown`. InternalVar are constructed by builder only.
  
  Comment: Internal Var can be used to represent diverse types such as enum
  type `DataType.FLOAT32`.
code: def __init__(self, val, name=None):
      super(InternalVar, self).__init__(
          name=name, sym_type=types.unknown, sym_val=types.unknown(val)
      )
  
imports: from coremltools.converters.mil.mil import types
  from coremltools.converters.mil.mil.types.symbolic import is_symbolic, any_symbolic
label_desc: Use image node_id 1 to create a new InternalVar object from inherited base classes: Var with example: obj = InternalVar(val, name)